Usually, a multitone method (DMT, discrete multitone, discrete multitone modulation) is used for an asymmetric data stream transmission via customary telephone lines, customary telephone lines being designed in a conventional [lacuna] as asymmetrical digital subscriber lines (ADSL).
One essential advantage of ADSL transmission techniques is that it is possible to use conventional cable networks for a transmission, pairs of copper cores that are twisted together usually being used.
Digital high-speed subscriber lines according to the prior art are described see for example in the publication “High-speed digital subscriber lines, IEEE Journal Sel. Ar. In Comm., Vol. 9, No. 6, August 1991”.
The known transmission methods with a high data rate based on digital subscriber lines (DSL) include a number of VDSL (Very High Data Rate DSL) arrangements, wherein e.g. methods such as CAP (Carrierless Amplitude/Phase), DWMT (Discrete Wavelet Multitone), SLC (Single Line Code) and DMT (Discrete Multitone) can be used therefor. In the case of the DMT method, the transmission signal is provided from multiple sinusoidal or cosinusoidal signals, each individual sinusoidal or cosinusoidal signal being able to be modulated both in terms of amplitude and in terms of phase. The multiple modulated signals thus obtained are provided as quadrature-amplitude-modulated signals (QAM=Quadrature Amplitude Modulation).
FIG. 4 shows a conventional data stream transmitter, into which data 123 to be transmitted are input via a data input device 201. The data 123 to be transmitted are fed to a coding device 202, in which the data are first coded and then combined to form coded data blocks 125, a predeterminable number of bits to be transmitted being assigned to a complex number depending on the stepped nature. Finally, the coded data blocks 125 output by the coding device 202 are fed to an inverse transformation device 203.
In a conventional manner, the inverse transformation device 203 uses an inverse fast Fourier transformation (IFFT) to transform the data that are present in the frequency domain into the time domain, N samples of a transmitter signal being directly generated from N/2 complex numbers, all N samples hereinafter being designated as a discrete multitone symbol (DMT symbol; DMT=Discrete Multitone). In this case, the complex numbers may be provided as amplitude values of cosine and sine oscillations (real part and imaginary part) to be transmitted within a data block, the frequencies being distributed equidistantly in accordance with the following relationship:
                              f          i                =                  i          ·                      1            T                                                                        ⁢                              i            =            1                    ,          2          ,                      …            ⁢                                                  ⁢                          N              /              2                                          
In this case, T denotes a time duration for a transmission of a discrete multitone symbol and N denotes a number of samples for a discrete multitone symbol. By way of example, in a “downstream” mode, i.e. during a data transmission from at least one switching center to at least one subscriber, conventional ADSL-DMT methods employ 256 tones which can each be modulated in magnitude and phase as sine tones. In this case, the fundamental frequency is 4.3 kHz and the frequency spacing between successive tones is likewise 4.3 kHz. A frequency spectrum from 4.3 kHz (fundamental frequency) to (4.3 kHz+256×4.3 kHz)=1.1 MHz is thus transmitted. Each DMT symbol is thus represented by a sine tone which can be modulated in magnitude and phase, it usually being the case that, per symbol, a maximum of 15 bits are represented as a complex number. During a transmission of a multitone signal formed in this way, however, the problem arises that transient processes are brought about by the transmission channel, which may be designed as a twisted copper two-wire line, for example, said transient processes having decayed after M samples, for example.
In the transmitter device, after an inverse fast Fourier transformation (IFFT), the last M samples of a DMT symbol are attached to a block start, the following relationship holding true: M<N. This cyclic extension (cyclic prefix) makes it possible to simulate a periodic signal for the data stream receiver when the transient process caused by the transmission channel has decayed after M samples, it being possible to avoid a mutual interference between different DMT symbols, i.e. an intersymbol interference (ISI).
As a result, in conventional methods, it is possible to considerably reduce an equalization complexity in an equalization device arranged in the data stream receiver, since, after a demodulation of the receiving analog data stream 101 in the data stream receiver, it is only necessary to perform a simple correction with the inverse frequency response of the transmission channel in the correction device 112.
An essential disadvantage of a data transmission according to the ADSL method via copper lines, in the case of which multitone signals are transmitted, consists in the fact that long transient processes occur. In a conventional manner, the cyclic prefix is therefore extended in order to supply a periodic signal for the data stream receiver. However, the cyclic prefix must be kept small in relation to the DMT symbol length N, i.e. the following relationship must hold true:M<<N,since otherwise a reduction of the transmission capacity occurs in a disadvantageous manner.
In the case of the ADSL standard, a DMT symbol length of N=64 and a value of a cyclic prefix of M=4 are provided, by way of example, for a data transmission from a subscriber to a switching system. In order to limit a transient process to the cyclic prefix, in the case of the known method, in the preprocessing device arranged in the data stream receiver, a special equalization device for the time domain (TDEQ=Time Domain Equalizer) is provided in the form of an adaptive transversal filter which operates with a sampling rate Fs (for example 276 kHz in the switching center in the case of ADSL).
By virtue of the required restriction of the length of the cyclic prefix to M=4, for example, as mentioned above, in the case of conventional methods for transmitting an analog data stream 101, a transmission quality is disadvantageously impaired since there is a considerable intersymbol interference (ISI) even with the use of an equalization device in the data stream receiver.
In a disadvantageous manner, a customary transmission channel furthermore contains high- and low-pass filters in order to limit the bandwidth of the analog data stream to be transmitted, and in order to suppress an out-of-band noise in analog-to-digital and digital-to-analog converters, which may be designed as sigma-delta converters, for example.
What is disadvantageous, in particular, is that, in the event of an excitation of low-pass filters with DMT signals, transient processes occur which, in a frequency range, have considerable spectral components above the envisaged transmission signal band. At a sampling rate Fs of 276 kHz, for example, spectral components which cannot be eliminated by the equalization device arranged in the data stream receiver are produced as a result of convolution products in the transmission signal band. In a disadvantageous manner, said convolution products are contained as interference signals in the transmission signal band, thereby impairing a transmission quality.
A multitone signal generated in the time domain is subsequently transmitted in the form of DMT symbols in accordance with FIG. 4. In order to provide an analog transmitter signal 211, an analog-to-digital converter is provided for a conversion from a digital multitone signal 303 into the analog transmitter signal 211.
A further known data stream transmitter is illustrated in FIG. 5, in which case, in addition to the components illustrated in FIG. 4, a first filtering device 131′ and a second filtering device 132′ are arranged between the inverse transformation device 203 and the digital-to-analog converter 204.
FIG. 5 illustrates that a typical transmission channel contains high- and low-pass filters for a band limiting of channel transmission signals. As shown in FIG. 5, the discrete multitone symbol (DMT symbol) 208 is subjected to high-pass filtering in the first filtering device 131′ in order to obtain a filtered discrete multitone symbol 209′. This filtered discrete multitone symbol 209′ is subjected to low-pass filtering in the second filtering device 132′. The filtering devices 131′ and 132′ used for band limiting have the disadvantage that, in the event of an excitation with DMT symbols transient processes occur which limit a data transmission rate. Particularly when low-pass filters are used as filtering devices 131′ and 132′, respectively, significant spectral components which have an effect particularly in the case of a short cyclic prefix occur in the frequency range of the signal band. The time signal output by the second filtering device 132′ is finally transmitted after a digital-to-analog conversion in a digital-to-analog converter, transient processes inexpediently occurring.
A main disadvantage of the conventional method illustrated in the form of a block diagram in FIGS. 4 and 5 is that a high crest factor, i.e. a high ratio of peak value to root-mean-square value of the multitone symbol to be transmitted is present in the case of the form of modulation which is used and which entails generation of a multitone signal. Conventional measures proposed in the literature for reducing the crest factor relate only to a data transmitter, whereas the influence of an analog front-end (AFE) is not taken into account. In a disadvantageous manner, the measures for reducing the crest factor in the data transmitter are generally compensated for again by the analog front-end AFE, so that a crest factor reduction is eliminated, or, in an extreme case, the crest factor can even increase as a result of these measures, due to transient processes of various digital and analog filters.
In a disadvantageous manner, high peak values governed by a high crest factor require the provision of a transmission bandwidth which does not contribute to an improvement of a signal-to-noise ratio of the multitone signal.